The present invention relates generally to a radiography device and, more particularly, to a radiography device having a multiple row spiral groove bearing for an X-ray tube.
The X-ray tube has become essential in medical diagnostic imaging, medical therapy, and various medical testing and material analysis industries. Typical X-ray tubes are built with a rotating anode structure for the purpose of distributing the heat generated at the focal spot. The anode is rotated by an induction motor consisting of a cylindrical rotor built into a cantilevered axle that supports the disc-shaped anode target, and an iron stator structure with copper windings that surrounds the elongated neck of the X-ray tube that contains the rotor. The rotor of the rotating anode assembly being driven by the stator which surrounds the rotor of the anode assembly is at anodic potential while the stator is referenced electrically to the ground. The X-ray tube cathode provides a focused electron beam that is accelerated across the anode-to-cathode vacuum gap and produces X-rays upon impact with the anode.
In an X-ray tube device with a rotatable anode, the target has previously consisted of a disk made of a refractory metal such as tungsten, and the X-rays are generated by making the electron beam collide with this target, while the target is being rotated at high speed. Rotation of the target is achieved by driving the rotor provided on a support shaft extending from the target. Such an arrangement is typical of rotating X-ray tubes and has remained relatively unchanged in concept of operation since its induction.
Inner rotation bearings for use in a rotating anode x-ray tube device are well known in the prior art. One typical type of x-ray tube support bearing includes ball bearings positioned between an inner and outer race to provide bearing support for the assembly. Although such bearing designs are common, they are not without disadvantages.
It is possible for present bearing designs to transfer torque through the ball bearings to the outer race. This transfer of torque can result in the rotation of the outer race that may in turn contribute to chatter of the bearing assembly. This is highly undesirable. In addition, present designs with a stationary, or nearly stationary, outer race may result in high velocities of the ball bearings during operation. The combination of rubbing due to race rotation, chatter, and high ball velocities can result in high acoustic noise generation during operation. This is, of course, highly undesirable.
Considerable effort and time has gone into the advancement of systems to lubricate the ball bearings in such designs in an effort to reduce these negative characteristics. These advancements in lubrication, however, can come at the expense of an increase in cost of the bearing assembly. In addition, such lubrication systems often leave room for improvement in the reduction of ball speed, torque transfer, and chatter. Reductions in such characteristics are highly desirable as they may lead to reduced wear on the ball bearings, an increase in the life cycle of the bearings, a reduction in acoustic noise generation, and possibly an increased anode run speed of the tube.
Therefore, there is a need for an X-ray tube bearing assembly that reduces ball speed, reduces transfer torque, reduces chatter, reduces acoustic noise generation, and may allow for an increase in the anode run speed of the tube.
One approach that has been used to increase the performance of rotating anode X-ray devices is to replace ball bearing type bearing assemblies with a spiral groove bearing. Spiral groove bearings are typically used in X-ray tubes as a means to run the tube very quietly. The spiral groove is a hydrodynamic bearing that typically uses gallium as a fluid interface. However, these bearings are typically speed limited, as higher speed operations can lead to excessive turbulence of the liquid, higher heat generation, and higher torques that affect the spiral groove bearing performance.
Another approach to improving the performance of the bearing assembly is discussed in copending U.S. application Ser. No. 09/751,976, entitled xe2x80x9cMultiple Row X-Ray Tube Bearing Assemblyxe2x80x9d, filed Dec. 29, 2000, in which the use of multiple row x-ray tube bearings, as compared with a single row, is proposed. The introduction of an intermediate freely rotating inner race allows each bearing row to rotate independently of each other. This can reduce ball velocity, outer race rotation, rubbing, and chatter. This bearing assembly may also allow for increased anode speed runs.
It is thus highly desirable to design a system that incorporates all the benefits of a multiple row X-ray bearing assembly with a spiral groove type bearing.
The present invention incorporates at least one dual spiral groove intermediate race into an X-ray tube assembly.
The introduction of an intermediate race having an outer and inner spiral grooved surface reduces the relative velocities and increases the overall speed capability in the bearing assembly. This enables higher target (shaft) velocities and corresponding higher focal spot power while reducing heat generation and torque requirements. All of these factors are improved because torque and power do not scale linearly with speed.
Other objects and advantages of the present invention will become apparent upon the following detailed description and appended claims, and upon reference to the accompanying drawings.